inflected. But the nature of the substance which is given is a very
important element: if equal-sized particles of glass (which acts only
mechanically), of gelatine, and raw meat, are placed on the discs of
several leaves, the meat causes far more rapid, energetic, and widely
extended movement than the two former substances. The number of glands
which are excited also makes a great difference in the result: place a
bit of meat on one or two of the discal [page 234] glands, and only a
few of the immediately surrounding short tentacles are inflected; place
it on several glands, and many more are acted on; place it on thirty or
forty, and all the tentacles, including the extreme marginal ones,
become closely inflected. We thus see that the impulses proceeding from
a number of glands strengthen one another, spread farther, and act on a
larger number of tentacles, than the impulse from any single gland.
Transmission of the Motor Impulse. In every case the impulse from a
gland has to travel for at least a short distance to the basal part of
the tentacle, the upper part and the gland itself being merely carried
by the inflection of the lower part. The impulse is thus always
transmitted down nearly the whole length of the pedicel. When the
central glands are stimulated, and the extreme marginal tentacles
become inflected, the impulse is transmitted across half the diameter
of the disc; and when the glands on one side of the disc are
stimulated, the impulse is transmitted across nearly the whole width of
the disc. A gland transmits its motor impulse far more easily and
quickly down its own tentacle to the bending place than across the disc
to neighbouring tentacles. Thus a minute dose of a very weak solution
of ammonia, if given to one of the glands of the exterior tentacles,
causes it to bend and reach the centre; whereas a large drop of the
same solution, given to a score of glands on the disc, will not cause
through their combined influence the least inflection of the exterior
tentacles. Again, when a bit of meat is placed on the gland of an
exterior tentacle, I have seen movement in ten seconds, and repeatedly
within a minute; but a much larger bit placed on several glands on the
disc does not cause [page 235] the exterior tentacles to bend until
half an hour or even several hours have elapsed.
The motor impulse spreads gradually on all sides from one or more
excited glands, so that the tentacles which stand nearest are always
first affected. Hence, when the glands in the centre of the disc are
excited, the extreme marginal tentacles are the last inflected. But the
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